In a radio communication system in which a plurality of radio communication apparatus transmit signals to one another, amplifiers (also referred to as power amplifiers) for amplifying transmitted signals before emitting them from antennas are used. An amplifier may have a nonlinear amplification characteristic. That is to say, the relationship between the power level of a signal inputted to an amplifier and the power level of a signal outputted from the amplifier may not be linear. The nonlinearity of an amplifier causes distortion of a transmitted signal. This may lead to inter-channel interference caused by power leakage into adjacent frequencies or a deterioration in radio communication quality.
Therefore, in order to control distortion of a transmitted signal which has passed through an amplifier, a radio communication apparatus may include a pre-distortion type distortion compensator. Before a transmitted signal is inputted to an amplifier, a pre-distortion type distortion compensator (also referred to as a predistorter) adds distortion having a characteristic reverse to the amplification characteristic to the transmitted signal. This controls distortion of the transmitted signal which has passed through the amplifier and compensates for the nonlinearity of the amplifier.
For example, a distortion compensator compensates for distortion of a transmitted signal by the use of a compensation coefficient corresponding to a power level of the transmitted signal. Some distortion compensators store a plurality of compensation coefficients used for distortion compensation in LUTs (LookUp Tables), designate addresses corresponding to the power levels of transmitted signals, and read out compensation coefficients from the LUTs. Furthermore, some distortion compensators compare original transmitted signals before distortion compensation with feedback signals generated from transmitted signals which have passed through amplifiers, and update compensation coefficients so as to make the differences between them small.
In addition, the following distortion compensator is proposed. The frequency with which each address in an LUT is designated is monitored. An address proportional to a power level is converted to an actual address in the LUT so as to equalize the frequency of designation. As a result, there is no great difference in frequency of update among a plurality of compensation coefficients.
International Publication Pamphlet No. WO03/103163
Japanese Laid-open Patent Publication No. 2007-49251
By the way, with a distortion compensator the following method is possible. A compensation coefficient is stored in a storage area, such as an LUT, so that it will be proportional to, for example, an index value (such as a power value, an amplitude, or a logarithm of the power value) indicative of a power level. A compensation coefficient is selected based on the index value. With the method for selecting a compensation coefficient by the use of one index value, however, compensation accuracy changes according to a power level. Accordingly, it is difficult to efficiently improve compensation accuracy in a wide range of power levels.
For example, if a plurality of compensation coefficients are prepared so that they will be proportional to a power value and an amplitude, then the number of compensation coefficients assigned to a range of power levels lower than an average may be smaller than that of compensation coefficients assigned to a range of power levels higher than the average. As a result, compensation accuracy may deteriorate on the low power level side. Furthermore, if a plurality of compensation coefficients are prepared so that they will be proportional to a logarithm of a power value, then the number of compensation coefficients assigned to a range of power levels higher than an average may be smaller than that of compensation coefficients assigned to a range of power levels lower than the average. As a result, compensation accuracy may deteriorate on the high power level side.